Method of forming micro-porous glass fibers

ABSTRACT

This invention relates to a process for forming small particles of microporous glass having inter-connected pores suitable to filter tobacco smoke. It particularly relates to a method of melting a phase-separable borosilicate glass composition, especially alkali borosilicates, forming the glass into a workable form, especially fibers, at elevated temperatures, phase separating the glass by heat treatment at elevated temperatures below the miscibility temperature of the glass for a sufficient period to form a silica-rich phase and a substantially continuous borate-rich phase, cooling the phase-separated glass, sizing the phase-separated glass, and leaching the glass to remove a sufficient quantity of the borate-rich phase to form microporous articles having inter-connected pores.

United States Patent Hammel et al.

[451 Mar. 21, 1972 [54] METHOD OF FORMING MICRO- POROUS GLASS FIBERS[721 Inventors: Joseph J. l-Iammel, Pittsburgh, Pa.; John D. Mackenzie,Schenectady, N.Y.

[73] Assignee: PPG Industries, Inc., Pittsburgh, Pa.

[22] Filed: May 6, 1969 [21] Appl. No.: 822,325

Related US. Application Data [63] Continuation-impart of Ser. No.736,670, June 13,

1968, abandoned.

[52] US. Cl ..65/3l, 65/2 [51] Int. Cl ..C03c 25/06 [58] FieldofSeai-ch..65/3l,2; 117/124; 131/261, 131/262, 263, 264, 265, 266, 267

[56] References Cited UNITED STATES PATENTS 2,834,738 5/1958 Vincent..65/3l X 2,106,744 2/1938 Hood et al.. 2,313,343 3/1943 Jacob..13l/261R Primary Examiner-Arthur D. Kellogg Attorney-Chisholm andSpencer This invention relates to a process for forming small particlesof microporous glass having inter-connected pores suitable to filtertobacco smoke. It particularly relates to a method of melting aphase-separable borosilicate glass composition, especially alkaliborosilicates, forming the glass into a workable form, especiallyfibers, at elevated temperatures, phase separating the glass by heattreatment at elevated temperatures below the miscibility temperature ofthe glass for a sufficient period to form a silica-rich phase and asubstantially continuous borate-rich phase, cooling the phase-separatedglass, sizing the phase-separated glass, and leaching the glass toremove a sufficient quantity of the borate-rich phase to formmicroporous articles having inter-connected pores.

ABSTRACT 6 Claims, No Drawings BACKGROUND OF THE INVENTION Thisapplication is a continuation-in-part of U.S. application Ser. No.736,670, filed June 13, 1968, now abandoned, describing tobacco smokefilters comprised of microporous glass particles having interconnectingpores.

The present invention claims a novel process for forming particles ofmicroporous glass suitable for use as a tobacco smoke filter.

Patent application Ser. No. 736,670 describes a method of forming smallparticles of microporous glass wherein an alkali borosilicatecomposition is melted, formed in sheets, phase separated while in sheetform, and subsequently leached to form a sheet of glass havinginterconnecting micropores. To form particles of suitable size for usein tobacco smoke filters, the microporous glass sheets were crushed andsized. This method produced microporous glass particles suitable for useas tobacco smoke filters but was disadvantageous in consuming very longperiods of time for leaching of the glass in sheet form.

INVENTION It has now been discovered that microporous glass havinginterconnecting pores useful as tobacco smoke filters can beadvantageously formed by:

a. melting a phase-separable metal borosilicate glass composition,especially alkali borosilicate compositions,

b. forming the glass into a workable form at elevated temperatures, theworkable form being sheets, rods, tubes, or fibers, for example,

c. phase separating the glass by treatment at elevated temperatures,preferably from about 450 C. to about 750 C. or higher, but below themiscibility temperature of the glass, for a sufficient period of time toform a silica-rich phase and substantially continuous borate-rich phase,

d. cooling the phase-separated glass,

e. sizing the phase-separated glass to a desirable size, for example,granules, fibers, etc., having a minor axis length of 0.5 to 1,000microns and, preferably from 100 to 500 microns for granules, and about1 to 20 microns for fibers, and

f. leaching the glass to remove a sufficient quantity of the borate-richphase to form microporous particles having interconnecting pores.

1n U.S. Pat. No. 2,106,744, the leaching or porous glass sheets wasconducted in an acid bath of a temperature of approximately 98 C. for atleast 1 day for each millimeter of glass thickness. In the presentinvention, it has been found that sufficient leaching of the glass maybe conducted by treatment with an acid at temperatures between 70 and200C. in about 2 hours or less with leaching of fibers of about 0.5 toabout 20 microns diameter being accomplished in a few minutes, e.g., 5minutes or less. Leaching is accomplished by use of an inorganic acid inconcentrated form. Hydrochloric, nitric, and sulfuric acids arepreferred.

Typical compositions useful in the process of the instant inventioninclude metal borosilicates having generally less than about 20 percentby weight metal oxide, especially soda and potassia, about 60 to 90percent by weight silica, and to 40 percent by weight boric acid.Preferred compositions comprise about 5 to 10 percent by weight alkalimetal oxide, about to 30 percent by weight boric oxide, and about 70 to85 percent by weight silica. Such glasses may be formed from alkalimetals such as sodium, potassium, or lithium, from alkaline earth metalssuch as calcium, strontium, barium, and the like, and from other metalssuch as lead, zinc, titanium, and the like.

A typical process for forming small, that is, particles of less than1,000 microns in diameter, micro-porous glass particles suitable fortobacco smoke filters comprises melting a phaseseparable borosilicateglass composition, especially alkali borosilicate compositions, formingthe glass into a sheet or rod, and heat treating the glass withoutallowing it to cool to phase separate a silica-rich phase from aborate-rich phase of a substantially continuous nature. The heattreatment is preferably carried out at temperatures of about 450 toabout 750 C. for a period of one hour or more. Heat treating at highertemperatures for longer periods of time tends to agglomerate theborate-rich phase and, upon leaching, provides a structure having largerpores than articles treated at lower temperatures for shorter periods oftime. In any event, the heat treatment is carried out below themiscibility temperature of the glass. The miscibility temperature maybee readily determined by heating a translucent, phase-separated glassat successively high temperatures and observing the temperature at whichthe translucent appearance ceases, which indicates the presence of onlyone phase.

The heat treated glass is cooled and crushed to size, although it ispreferred to crush to a slightly larger size than desired for tobaccosmoke filter particles inasmuch as some fragmentation occurs during theleaching step. The glass is then leached in an acid which dissolves theborate-rich phase, for example, hydrochloric acid, sulfuric acid, orcombinations thereof, at temperatures above room temperature for aperiod sufficient to remove substantially all the borate-rich phase.

The resulting microporous particles having interconnecting pores of adiameter of about 10 angstroms to about 1,000 angstroms are thoroughlywashed to remove trapped acids and residue.

The method of this invention is particularly useful for formingmicroporous glass fibers uniquely useful in tobacco smoke filters. Aphase-separable glass composition such as the borosilicates, especiallyalkali metal borosilicates, preferably containing about 5 to 10 percentby weight alkali metal oxide, especially soda and potassia, 15 to 30percent by weight boric oxide, and about 70 to percent by weight silica,is melted and fritted. The fritted glass is then remelted, generally attemperatures of about l,204.4 C. to about 1,482.2 C. although higher orlower temperatures may be utilized inasmuch as the melting temperaturewill vary with glass composition. The molten glass is held at themelting temperature for a period of time sufficient to fine the glass,generally about 20 minutes to about 2 hours. The temperature of theglass is then lowered so that its temperature at the bushing providesthe proper viscosity for drawing of fibers. The fiber drawingtemperature will generally range from about 27.8 C. to about l94.4 C.below the melting temperature of the glass. For the above preferredcomposition, a drawing temperature of about 1,260 C. i 27.8 ispreferred. Glass fibers are then drawn through the bushing at the bottomof the crucible containing the molten glass.

The size of fibers drawn depends upon the drawing speed which ispartially dependent upon temperature since temperature affects viscosityand the bushing orifice is of a larger diameter than typical fiberdiameters. A typical temperature range for drawing metal borosilicateglasses ranges from about 982.2 C. to about l,648.9 C. and, preferably,from about 1,148.9" C. to about 1,3 15.6 C. The metal borosilicateglasses of this invention have a flat viscosity curve over a boardtemperature range, thereby permitting drawing of fibers over a widetemperature range. Drawing speeds at these temperatures may range from2,000 feet per minute to 20,000 feet per minute, preferably from about5,000 feet per minute to 15,000 feet per minute, with fiber diametersranging from 0.5 microns at the faster drawing speeds to about 20microns at slower rates of draw.

If the glass drawn is too seedy to allow drawing of fine fibers, then athicker fiber, e.g., 20 microns, may be drawn and then attenuated to asmall diameter, e.g., 0.5 to 5 microns, by drawing through a flame.Other attenuating processes involving steam, hot air, etc., are alsouseful.

A flame temperature of l,648.9 to 2,093.3 C. is preferred for flameattenuation at a fiber rate of about 600 to 1,000 feet per second. Thisapproach has proved useful in the instant invention in forming smalldiameter fibers from compositions which do not contain fining agents oralkaline earth oxides.

Fibers of phase-separable glasses, especially borosilicate glassescontaining alkali metal oxides, alkaline earth oxides, or oxides oflead, zinc, titanium, zirconium, and the like, may be heat treated atelevated temperatures to induce phase separation. Temperatures fromabout 450 C. to about 750 C., preferably about 550 C. to about 680 C.,have been found useful for this purpose. The period of heat treatment toinduce phase separation for fibers may range from less than about 3minutes to about 4 hours or more, with the longer time periods requiredat the lower temperatures. For example, at temperatures of about 627 C.,heat treatments of about 5 to 30 minutes were found to phase separatethe glass very satisfactorily.

Fibers may be preferably cut into lengths of a few inches to severalfeet or more and formed into bundles or mats for heat treatment. Suchbundles or mats can be easily formed in a water bath although drybundling is effective. A loose arrangement such as bundles or matsprovides better heat transfer to each fiber than would be achieved inclose packing such as occurs on spools. Also, leaching proceeds at amuch faster rate for loosely packed fibers. It is significant that thefibers do not fuse together at the heat treating temperatures inasmuchas U.S. Pat. No. 2,461,84l indicates that fibers of alkali borosilicateglasses cannot be treated at such temperatures.

Leaching may be preferably conducted for fibers by treating inconcentrated inorganic acids such as hydrochloric, sulfuric, and nitric,i.e., a normality of from about 1 to about 6, for a period of about 3minutes to about 60 minutes, at a temperature of from about 70 C. toabout 200 C.

Fibers heat treated and leached in the above-described manner aremicroporous in nature having interconnecting pores of about 40 angstromsin diameter to about 1,000 angstroms. The larger pore diameters requirelonger heat treatment at temperatures in the upper portion of the heattreatment range. Pore diameters of about 40 angstroms to about 250angstroms may be obtained readily by heat treating at about 570 C. toabout 630 C. for about 5 minutes to about 300 minutes.

Microporous fibers produced in the above-described manner have surfaceareas generally in excess of 100 square meters per gram as determined bynitrogen absorption according to the method described by Emmett,Brunauer, and Teller in Journal of the American Chemical Society, 56, 35(1934) and 57, 1954 (1935).

These fibers have a pore volume of from about 5 percent to about 30percent, depending upon composition, degree of phase separation, andleaching conditions. A pore volume of to percent is readily attainablewith the above-described compositions and processing conditions.

Although the above description emphasizes processing of individualbundles or mats of fibers, and especially thin mats of about 2 milthickness, continuous treatment of continuous strands of fibers isfeasible and for some operations may be preferable. Fibers may be drawnat high speeds onto spools; fibers drawn from many spools may be fed instrands at a desired rate through a heat-treating furnace and into aleaching bath in a continuous manner. The fibers can then be washed andagain placed on spools or fed directly into an apparatus for bundling,cutting, and assembling of the fibers into filters for cigarettes,pipes, and the like.

Fiberizing is an especially effective method of forming microporousglass tobacco smoke filters from alkaline earth borosilicates andborosilicates of lead, zinc, titanium, zirconium, and the like, sincethese glasses tend to phase separate upon cooling unless the cooling isextremely rapid. The most effective means of cooling occurs when fine,i.e., about 0.5 to about 20 microns thick, fibers are drawn. Because ofthe thinness of the fibers, the whole mass of the fiber reaches ambienttemperature very rapidly.

Glasses which phase separate upon cooling have been found generally tohave fewer interconnecting pores than glasses which are phase separatedby a controlled heat treatment. The quantitative presence ofinterconnecting pores can be determined readily by measuring the weightloss rate during leaching, and by the appearance of the glass under anelectron microscope and nitrogen area measurements of interconnectingpores. If a phase-separated glass does not possess an interconnecting orcontinuous phase, weight is lost very slowly during leaching and thetotal weight loss is considerably less than in phase-separated glasseshaving a continuous phase.

Fiberizing of alkaline earth borosilicates and borosilicates containinglead, zinc, titanium, zirconium, and the like, provides fibers of onephase which may then be phase separated by controlled heat treatment toobtain a glass fiber having a silica-rich phase and a continuousborate-rich phase. The heat treating can be conducted as describedabove. The resulting phase-separated fiber may be leached in the mannerdescribed hereinabove to give a microporous glass having interconnectingpores ofa diameter of about 40 angstroms to about 1,000 angstroms.

Other glass-forming ingredients, fluxes, and fining agents and the likemay be included in the glasses treated according to the instantinvention; however, such additions should be made only in quantitieswhich do not adversely affect phase separation or leaching. Generally,minor quantities of alumina and P 0 may be present without introducingan adverse result. Also, typical fining agents such as Sb O NaCl, andthe like may be included, if desired, without altering thecharacteristics of the glass.

This invention provides for rapid leaching of very small particles orfibers of a phase-separated glass having a continuous, leachable phase.The invention further provides a fiberizing process for forming veryeffective microporous tobacco smoke filters.

EXAMPLE I Porous Fiber Glass Filter Oxide Weight Percent Na,o 5 B 0 20SiO 75 Raw materials for this glass were melted and fritted. The frittedglass was then remelted at about 2,800 F. in a small crucible above abushing having an orifice of about 6 microns diameter. The glass washeld at about 2,800 F. for about 1 hour before fibers were drawn. Fiberdrawing conditions were a drawing speed of 5,000 feet per minute at abushing temperature of l,287.8 C. during one run and a speed of 5,500feet per minute at l,32l.1 C. bushing temperature during another run.The fibers produced by both runs were substantially identical, having adiameter of about 6 microns.

The fibers were cut into lengths of about 6 to 9 inches and spread outinto a mat for heat treating. Heat treating was conducted at 575 C. for5 hours to bring about phase separation. The fibers were then cooled toroom temperature and leached in 3 normal hydrochloric acid at 98 C. forabout 1% hours. The fibers were then thoroughly washed with water.

The resulting fibers had interconnecting pores of about 75 angstroms indiameter, as determined from electron micrographs, and a chemicalcomposition of about percent by weight SiO 5 percent by weight B 0 and0.1 percent by weight Na O. The pore volume was between 20 and 25percent. The surface area of the porous fibers was about 137 squaremeters per gram as determined by nitrogen absorption according to themethod described by Emmet, Brunauer, and Teller in Journal of theAmerican Chemical Society 56, 35 (1934)and57, 1954(1935).

The effectiveness of these porous fibers as tobacco smoke filters wastested by preparing Winston cigarettes with a loosely packed porousfiber glass filter element 20 millimeters in length. These cigaretteswere compared with a regular cellulose acetate filter and a non-porousfiber glass filter of fiber glass which had not been phase separated andleached. These filters were also 20 millimeters in length. The followingtable shows the results:

1 21116 pressure drop shown is across the entire cigarette, including.Av e r a ge of cigarettes for each type of filter. 3 Regular,non-porous fiber glass of the same composition without phase separationand leaching has a surface area of about 6 mfl/gm.

The porous fiber glass proved to be significantly more effective inremoving tars and nicotine than the commercial grade cellulose acetatefilter and was the non-porous fiber glass (non-porous) filter. Regularnon-porous fiber glass has a nitrogen surface area of about 6 squaremeters per gram while the porous fiber glass utilized in this examplehad a nitrogen surface area of about 137 square meters per gram.

EXAMPLE ll The following table sets forth the results of fiberssubmitted to various heat treating conditions and leaching conditions.The fibers were prepared from a glass having a calculated composition of75 percent by weight SiO 20 percent by weight B 0 and 5 percent byweight Na O. The fibers were melted and drawn in the method described inthe above example and had an average diameter of about 6 microns.

Many of the samples were prepared by matting in water or methanol. Thiscomprises placing short lengths, e.g., 3 inches to 24 inches, of fibersin a container of water or other liquid, agitating mildly, and drawingoff a mat of the fibers. The mat -of fibers utilized in the aboveexamples generally had a thickness of about 2 mils. Several of thesamples, for example, samples 5 and 7, were not matted. Sample 6 wasmatted in methanol to determine if any leaching of the alkali in theglass fiber occurred during the water-matting stage. It is noted thatthe weight loss was greater for the methanol-matted sample than for thewater-matted samples.

Although the following table does not indicate, the samples were driedafter being washed with water and before being weighed. Also, thewater-matted and methanol-matted samples were dried thoroughly beforebeing weighed.

The composition of the phase-separated, leached fibers, that is, allsamples other than samples 8 and 9, had a composition of about 96percent by weight silica, 3.8% B 0 and 0.2% Na O. Samples which had thelowest weight loss would be slightly richer in 13 0 and Na O while thosesamples having the greatest weight loss would contain less B 0 and Na O.

The heat treatment was conducted in a small furnace which was heated tothe temperature indicated. A door of the furnace was then opened and thesample was placed in the furnace for the period of time indicated. Thus,the actual time the sample spends at the heat treating temperature isslightly less than that indicated above. At higher temperatures than 627C. and with more efficient heating means, a phase-separation time ofless than 5 minutes is possible.

Samples 8 and 9 were leached without any prior treatment to illustratethe difference in weight loss between a sample which has not been phaseseparated into an acid-soluble, borate-rich phase. The weight loss ofthese two samples is primarily boric oxide and soda present near thesurface of the fiber.

All of the above samples, except samples 8 and 9, had interconnectingpores having a diameter greater than about 40 angstroms after heattreating and leaching.

EXAMPLE Ill The following table sets forth the results of fiberssubmitted to various heat treating and leaching conditions. The fiberswere prepared from a glass having a calculated composition of aboutpercent by weight SiO 20 percent by weight B 0 and 5 percent by weightNa O. The fibers were melted and drawn in the method described inExample I and had an average diameter of about 6 microns.

TABLE 11] Heat Treatment Leaching Sample Weight Time Temp. Time Temp.Weight Loss Sample (min.) (C.) tmin.) (C.) (grams) (2) l 1 min. 650 3min. 100 0.1248 19.5 2 2 min. 650 3 min. 98 0.1466 18.7 3 3 min. 650 3min. 97 0.1071 20.8 4 4 min. 650 3 min. 0.1443 20.0

5 3 min. 95 0.1064 1.9 6 1 min. 650 1 min. 0.1260 8.6 7 2 min. 650 1min. 98 0.1390 12.5 8 2 min. 650 2 min. 97 0.1444 15.3 9 2 min. 650 3min. 96 0.1415 19.5

In the above table, all samples were prepared by matting short lengthsof fibers, e.g., 3 inches to about 24 inches, in water and drawing off amat of about 2 mils (50 microns) in thickness. The samples were thendried at room temperature before being inserted into a furnace for heattreating.

The times recorded above represent the residence time in the heattreating furnace or leaching bath. The fibers have excellent thermalefficiency inasmuch as a residence time of 1 minute in furnace at afurnace temperature of 650 C. causes considerable phase separation.Since some finite time is required for the fiber to attain a minimumphase-separation temperature, which is preferably about 450 C., thefibers could be heat treated for even short periods of time where moreefficient heating means were used, e.g., the use of the TABLE II (3NH01) Sample Heat treatment Leaching washing weight time, (Before WeightTime, Temp, Time, Temp, cold H10, leac loss, Sample Preparation hrs. 0.min. 0. min. ing), g percent 1 H 0 matted (2 mil) 50 cc" 4 580 5 98 100. 285 22.4 2.. ..dc 4 580 10 94 15 0. 383 24.1 3 4 580 20 95 20 0. 45225. 8 do 4 580 34 98 20 0. 404 27. 0 No H1O mat, fiufl. 24 580 10 98 20O. 248 24. 5 Methanol matted (2 5 580 10 98 20 0. 343 30. 5 No H 0 mat,flufi 5 580 10 97 20 0. 269 26. 1 H2O matted (2 mi None None 14 97 20 0.312 2. 1 None None 10 95 10 0. 341 4. 2

Time, min.

No'lE.-Glass melted and drawn under same conditions described in firstpatent ap lication. Glass composition before leaching 75% S101, 20%B203, and 5% NazO; after leaching, 96% SiOz, 3.8 0 B 0 and 0.2% N330,

fiber itself as an electrical resistor, or where higher furnacetemperatures were utilized to effect faster heat transfer. Single fibersor strands of fibers could be drawn through a furnace or heatedelectrically in a continuous manner without prior cutting, matting, etc.

After leaching in 3N sulfuric acid at the times and temperaturesindicated, all samples were washed in water for 5 minutes and driedbefore weighing.

Sample No. 5 was not heat treated prior to leaching. The very low weightloss by leaching indicates that leaching proceeds slowly if the sampleis not first phase separated. Also, even if a non-phase-separated sampleis leached for a sufficient time to remove to percent or more of thesample weight, the finished product is much different in structure.Because of the random nature of leaching in non-phaseseparated fibers,the leached fibers generally have little strength and easily crumbleunless subsequently heat treated to form a substantially non-porous,high-silica material. Also, unless the fibers are first heat treated toform a minor acidsoluble, substantially continuous phase, the leachedproduct will not contain interconnecting pores.

The fibers set forth in the above table, except sample No.5, afterleaching had interconnecting pores of a diameter greater than about 40angstroms.

Fibers formed of compositions having potassia substituted for soda inthe above example behave similarly upon treatment to yield microporousfibers having interconnecting pores. Furthermore, the addition of analkaline earth oxide such as BaO, CaO, or the like to a composition suchas utilized in this example improves the fiber-forming properties.

Nitrogen surface areas of the microporous glass utilized in thisinvention were determined according to standard techniques. Suchtechniques are described by S. Brunauer in The Absorption of Gases andVapors, Vol. I, p. 271, Oxford University Press, London 1954).

Methods for determining the quantitative presence of hydroxyl groups onglass surfaces, especially porous glass surfaces, have been described byM. J. D. Low and N. Ramasubramanian, Infrared Study of the Nature ofHydroxyl Groups on the Surface of Porous Glass, Journal of PhysicalChemistry, Vol. 70, No. 9, pp. 2,740-2,746, Sept. 1966. The presence ofhydroxyl groups on porous glass utilized in this invention wasdetermined according to the techniques described by Low andRamasurbramanian.

Techniques for determining components in cigarette smoke have beendescribed by R. J. Phillippee, H. Moore, R. G. Honeycutt, and J. M.Ruth, Some Hydrocarbons of the Gas Phase of Cigarette Smoke, AnalyticalChemistry, Vol. 36, No. 4, pp. 859-865, Apr. 1964, and by R. J.Phillippee and M. E. Hobbs, Some Components of the Gas Phase ofCigarette Smoke, Analytical Chemistry, Vol. 28, No. 12, pp. 2,0022,006,Dec. 1965.

Pore size reported hereinabove was determined by photographing a surfaceof the microporous glass through an electron microscope, physicallymeasuring the pores on the photograph, and then dividing the measurementby the magnification of the microscope.

EXAMPLE lV Sample Acelaldehyde Acetone Winston cigarette with 20 mm.

standard cellulose filter 78 20 Winston cigarette with 20 mm.

porous fiber glass filter 66 12 The porous fiber glass removed about 15percent more acetaldehyde and about 40 percent more acetone than thecellulose acetate filter.

While specific examples have been set forth hereinabove to illustratethe invention, the invention is not to be considered limited thereto,but to include all the variations and modifications falling within thescope of the appended claims.

We claim:

1. The method of forming microporous fibers from a phaseseparableborosilicate glass comprising a. melting said glass,

b. drawing the glass at elevated temperatures into fibers of about 0.5to about 20 microns diameter,

c. phase separating the glass at an elevated temperature below themiscibility temperature of the glass in the range of from about 450 C.to about 750 C. for a sufficient time period to form a silica-rich phaseand a substantially continuous borate-rich phase,

d. cooling the phase-separated glass, and

e. leaching the glass to remove a sufficient quantity of the borate-richphase to form microporous fibers having interconnecting pores.

2. The process of claim 1 wherein the glass is an alkali metalborosilicate composition.

3. The process of claim 2 wherein the glass additionally contains analkaline earth oxide.

4. The process of claim 1 wherein the phase separated glass is leachedin acid to remove the borate-rich phase thereby forming a microporousstructure comprising in excess of 90 percent by weight silica and havinginterconnecting pores ofa minimum diameter of about 40 angstroms.

5. The process of claim 2 wherein the phase separable alkali metalborosilicate glass comprises less than about 20 percent by weight alkalimetal oxide, about 10 to about 40 percent by weight boric acid, andabout 70 to about percent by weight silica.

6. The process of claim 5 wherein the alkali metal oxide is soda.

2. The process of claim 1 wherein the glass is an alkali metalborosilicate composition.
 3. The process of claim 2 wherein the glassadditionally contains an alkaline earth oxide.
 4. The process of claim 1wherein the phase separated glass is leached in acid to remove theborate-rich phase thereby forming a microporous structure comprising inexcess of 90 percent by weight silica and having interconnecting poresof a minimum diameter of about 40 angstroms.
 5. The process of claim 2wherein the phase separable alkali metal borosilicate glass comprisesless than about 20 percent by weight alkali metal oxide, about 10 toabout 40 percent by weight boric acid, and about 70 to about 85 percentby weight silica.
 6. The process of claim 5 wherein the alkali metaloxide is soda.